Great whites' diets shown to vary widely

SCIENCE

Updated 11:00 pm, Tuesday, October 16, 2012

This 1,700-pound great white, caught by a fisherman in Morro Bay, was among the sharks examined for the study.

This 1,700-pound great white, caught by a fisherman in Morro Bay, was among the sharks examined for the study.

Photo: Tim Stephens, UC Santa Cruz

Great whites' diets shown to vary widely

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It may not come as a surprise that great white sharks are not picky eaters, but a team of researchers at UC Santa Cruz has found that they are not simply gluttons for seals, sea lions and those odd hairless mammals that flap around in the surf.

The study, published recently in the scientific journal PLoS One, showed that the dining habits of the finned creatures vary widely and often include dolphins, squid and a wide variety of fish.

"Some sharks are very specialized," said Sora Kim, a former UC Santa Cruz graduate student who led the study. "Others are more willing to eat whatever is around, whatever is easiest to get."

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The researchers analyzed growth bands in the vertebrae of 14 adult white sharks caught along the west coast of California and one caught in Baja California between 1957 and 2000 and traced their diets over the course of their lives using carbon isotopes and nitrogen embedded in the tissues.

They determined that different sharks eat different things and that their appetites change over time, kind of like asparagus-loving adults who hated all vegetables when they were young.

Kim, who now works as a postdoctoral researcher at the University of Wyoming, said at least one of the toothy predators in the study appeared to have specialized almost exclusively on fish and squid. Most, though, had a mix, probably a reflection of the fact that sharks only spend part of the year close to shore. Marine mammals appear to be repast for sharks along the coast while squid and fish go easiest down the gullet out at sea, she said.

The good news is that people - even surfers and boogie boarders - are not generally on the menu, at least not according to the tissue samples Kim examined.

The testing showed that preferences changed as the sharks grew older and differed among individuals, said Paul Koch, professor of earth and planetary sciences at UC Santa Cruz.

"We confirmed that the diets of many individuals observed at seal and sea lion rookeries shift from fish to marine mammals as the sharks mature," said Koch, a co-author of the study.

The study is important because it gives scientists a better idea of what great whites do offshore and how much variation there is in their behavior. There are believed to be only about 340 white sharks that frequent the Pacific coast between California and Guadalupe Island, in Mexico.

Great white sharks, known scientifically as Carcharodon carcharias, average 15 to 16 feet in length but can grow to 20 feet. The biggest one on record, caught in 1939, was 21 feet long and weighed 7,300 pounds.

A Stanford University-led shark-monitoring program, called Tagging of Pacific Predators, or TOPP, recently determined that this region's sharks, known as northeastern Pacific white sharks, are genetically unique compared with other great whites around the world.

The electronic monitoring showed how the northeastern Pacific population moved back and forth from a deep ocean spot near Hawaii known as the White Shark Cafe to Northern California. Most of the sharks are currently hanging around the Red Triangle, an area roughly between Monterey Bay, the Farallon Islands and Bodega Head, where they will feed mostly on seals and sea lions until early winter.

It was known, based on the stomach contents of captured great whites, that they scavenged a bit. Turtles, birds and human remains have been found inside them, but Kim said they actually have a wider diet than most scientists gave them credit for.

One curious discovery during the recent study was that shark diets changed slightly as marine mammal populations increased after the passage of the Marine Mammal Protection Act in 1972.

"We just really didn't have a very good understanding of shark diet," she said. "We just assumed they ate seals and sea lions because we can observe that, so that was our idea of what their diet was. Turns out they are just like humans in that different people like to eat different things."

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